1. Field of the Invention
The present invention relates to an aperture unit that adjusts the amount of light that passes through an imaging system, and to a lens barrel including the aperture unit.
2. Description of the Related Art
Image pickup apparatuses, such as digital cameras and video cameras, include an aperture unit that adjusts the amount of light that passes through an imaging system when taking an image. An example of such aperture unit is a so-called iris aperture unit that changes the shape of an aperture through which light passes while maintaining a substantially circular shape of the aperture by rotating a plurality of diaphragm blades using a member that rotates around the optical axis. With the iris aperture unit, an aesthetically pleasing blur can be realized in a taken image, because the aperture of the iris aperture unit has a substantially circular shape.
However, because the iris aperture unit includes a large number of components, an apparatus including the iris aperture unit tends to be large.
Another example is a guillotine-type aperture unit that changes the shape of an aperture by moving two diaphragm blades in opposite directions between an open aperture state and a closed aperture state. The guillotine-type aperture unit, which has a smaller number of components and has a simple structure, is widely used in consumer-oriented cameras and other apparatuses. However, the aesthetic quality of blur of an image taken by using the guillotine-type aperture unit tends to be lower than that of an image taken by using the iris aperture unit, because the aperture of the guillotine-type aperture unit has a substantially rhombic shape.
Regarding a mechanism for driving diaphragm blades, Japanese Patent Publication No. 6-3520 and Japanese Patent Laid-Open No. 10-301159 each disclose a guillotine-type aperture unit including three or more diaphragm blades. The aperture of such an aperture unit has a polygonal shape, whereby the aesthetic quality of the blur is improved compared with the case where the aperture has a substantially rhombic shape.
Japanese Patent Publication No. 6-3520 describes an aperture unit having a pentagonal aperture formed by three diaphragm blades and an aperture unit having a hexagonal aperture formed by four diaphragm blades. Japanese Patent Laid-Open No. 10-301159 also describes an aperture unit having a hexagonal aperture formed by four diaphragm blades.
When three or more diaphragm blades are used in a guillotine-type aperture unit so as to form a polygonal aperture, the movement distances of the diaphragm blades are larger than those when two diaphragm blades are used. In the guillotine-type aperture unit, sides of the polygonal aperture (aperture edges) are formed by using a combination of diaphragm blades that move larger distances and diaphragm blades that move smaller distances. As compared with the diaphragm blades of an aperture unit having a rhombic aperture, the diaphragm blades that move larger distances have smaller vertex angles at the aperture, and the distances between the optical axis and the vertices of such diaphragm blades are large in an open aperture state. When the aperture unit also functions as a shutter, the overlap amount of diaphragm blades in a closed aperture state are to be large in order to prevent double exposure due to rebound of the shutter. Therefore, the movement distances of the diaphragm blades are to be increased. In order to move the diaphragm blades by larger distances, the size of the aperture unit has to be increased. Moreover, the diaphragm blades are moved by using a long arm, which is difficult to actuate. Thus, in order to reduce the size of an aperture unit, it is important to minimize the movement distances of the diaphragm blades.
Diffraction of light physically occurs at the aperture. When the aperture is small, diffraction of light significantly influences the optical performance, so that the imaging performance is decreased. Therefore, except when taking an image of a specific object, an aperture value that has a small influence on the optical performance is normally used. To further reduce the amount of light, an ND filter is inserted in the optical path to adjust the amount of light. A small aperture state may be employed, for example, when taking an image of an object in full sunlight. However, because the optical performance is decreased under such a circumstance due to the influence of diffracted light, a strictly polygonal aperture for producing an aesthetically pleasing blur is not necessary.
Therefore, it is important that the shape of the aperture maintains a similar polygonal shape at least in the range from the open aperture to a normally used aperture value. Moreover, it is important to appropriately set the shape of the aperture formed by the diaphragm blades so as to reduce the movement distances of the diaphragm blades.